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1.
Influence of unsteady wake on a turbulent separation bubble 总被引:1,自引:0,他引:1
An experimental study was made of turbulent separated and reattaching flow over a blunt body, where unsteady wake was generated
by a spoked-wheel type of wake generator with cylindrical rods. The influence of unsteady wake was scrutinized by altering
the rotation direction (clockwise and counter-clockwise) and the normalized passing frequency (0 ≤ St
H
≤ 0.20). The Reynolds number based on the cylindrical rod was Re
d
=375. A phase-averaging technique was employed to characterize the unsteady wake. The effect of different rotation directions,
which gave a significant reduction of x
R
, was examined in detail. The wall pressure fluctuations on the blunt body were analyzed in terms of the spectrum and the
coherence.
Received: 15 January 2001 / Accepted: 17 July 2001 相似文献
2.
Using a normal mode analysis, we predict the infinite dilution linear viscoelastic properties of single-strand (ss) DNA molecules
and compare the results to the linear viscoelastic data of Shusterman et al. (Phys Rev Let 92(4):048303, 2004) obtained by
monitoring the diffusion of a fluorescently labeled terminus of the molecule. To compute the overall best global fit, we constrain
the hydrodynamic interaction parameter, h*, equilibrium root mean square spring extension, b, and the number of Kuhn steps per spring, N
K,S, to be equal for the strands compared. The fits using the bead-spring model for all but 23,100 base ss-DNA strands match
the experimental data at long times with significant deviations at intermediate and short times. However, parameters fitted
separately to all individual strand lengths predict results well. The best fits to data for 2,400 and 6,700 base pairs yield
N
K,S ∼12 and h* = 0.12. These values are similar to those found for conventional polymers such as polystyrene which have been successfully
modeled with N
K,S ∼7 and h* = 0.15, indicating ss-DNA and polystyrene exhibit analogous hydrodynamic behavior. 相似文献
3.
A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed
annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb
2/ν= 9 .5 × 104 (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary
layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity
measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence
by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326,
2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural
parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et
al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events
in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major
source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326,
2001). 相似文献
4.
V. M. Kulik 《Experiments in fluids》2001,31(5):558-566
Change of drag reduction (DR) along a tube (D=2 mm, L=4 m) was experimentally investigated. To attain turbulent flow with Re=8 × 104, a tank operated under high pressure up to 16 MPa. Solutions of different brands of polyethyleneoxide (PEO) with concentrations
from 1 ppm to 100 ppm were tested. The results indicate that DR is not a constant value but depends on the time and intensity
of interaction between the polymer and the turbulent flow. There are three regions with different behaviors of DR: growth,
maximum, and slope down. Maximum DR coincides with the Virk ultimate DR and can be described by the suggested simple formula
. A decrease in the DR maximum has not been found even for high shear stresses τ
p < 800 Pa. DR dynamics for four brands of PEO with different molecular weight was studied. Direct experimentally determined
DR may be greater than the Virk ultimate value if the change in velocity profile is not taken into account. The corrected
DR never exceeds the ultimate DR.
Received: 10 April 2000/Accepted: 24 May 2001 相似文献
5.
Ki Byung Sunwoo Seung Joon Park Seong Jae Lee Kyung Hyun Ahn Seung Jong Lee 《Rheologica Acta》2002,41(1-2):144-153
Three-dimensional numerical simulation of viscoelastic coextrusion process has been performed and numerical results were
compared with the experimental data of Karagiannis et al. (1990). By varying the magnitude of the second normal stress difference
and its ratio of Fluid I and Fluid II, we were able to control the interface profile and the degree of encapsulation along
the downstream direction. By increasing the parameter α (αFluid I=αFluid II) from 0.1 to 0.4 in the Giesekus model and increasing the α ratio (αFluid IαFluid II) between Fluid I and Fluid II from 2.0 to 4.0 in the permissible range of realistic polymeric systems, the interface profile
and the degree of encapsulation along the downstream direction were fitted with the experimental results. There was little
difference between the numerical results and the experimental data in the interface profile and the degree of encapsulation
along the downstream direction when the α ratio was set to 3.0 (0.3:0.1). Fluid I with larger magnitude of the second normal
stress difference protrudes into Fluid II with smaller magnitude of the second normal stress difference around the symmetric
plane, while Fluid II wraps around Fluid I near the side walls. As the ξ
1
ratio (ξ
1
,Fluid Iξ
1
,Fluid II) increases from 1.0 to 3.0 for the two-mode Phan-Thien and Tanner model, it was found that the curvature of the interface
profile increased, and the difference between the numerical results and the experimental data in the interface profile and
the degree of encapsulation along the downstream direction was almost negligible when the ξ
1
ratio was set to 3.0 (0.54:0.18). Although the parameters of viscoelastic models were fitted by using the shear viscosity
data only, quantitative agreements between the numerical results and the experimental coextrusion data were quite satisfactory.
Received: 24 April 2001 Accepted: 5 June 2001 相似文献
6.
Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined
wall of the diffuser is chosen as 8.5°, a case for which recent experimental data are available. Reasonable agreement between
the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall
inclined at a diverging angle of 10°, which has served as a test case for a number of experimental as well as numerical studies
in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky
model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus
the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of
the inflowing, fully turbulent channel flow has been studied, in particular, Re
b
= 4,500, Re
b
= 9,000 and Re
b
= 20,000 with Re
b
being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing
the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number
regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour
as the main separation region, i.e. the flow is separating less with higher Re
b
. Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent
study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for
larger Re
b
. Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow
condition (similar to a precursor simulation) for the diffuser is assessed. 相似文献
7.
M. D. Brodetsky A. M. Kharitonov E. Krause A. A. Pavlov S. B. Nikiforov A. M. Shevchenko 《Experiments in fluids》2000,29(6):592-604
The leeside vortex structures on delta wings with sharp leading edges were studied for supersonic flow at the Institute of
Theoretical and Applied Mechanics of the Russian Academy of Sciences in Novosibirsk. The experiments were carried out with
three wings with sweep angles of χ=68°, 73°, and 78° and parabolic profiles in the 0.6 × 0.6 m2 test section of the blow-down wind tunnel T-313 of the institute. The test conditions were varied from Mach numbers M=2 to 4, unit Reynolds numbers from Re
l=26 × 106 to 56 × 106 m−1, and angles of attack from α=0° to 22°. The results of the investigations revealed that for certain flow conditions shocks
are formed above, below, and between the primary vortices. The experimental data were accurate enough to detect the onset
of secondary and tertiary separation as well as other boundaries. The various flow regimes discussed in the literature were
extended in several cases. The major findings are reported.
Received: 6 September 1999/Accepted: 24 January 2000 相似文献
8.
An experimental study is presented for water droplet impingement on a liquid surface. The impaction process was recorded
using a high-speed digital camera at 1,000 frames/s. The initial droplet diameter was fixed at 3.1 mm ± 0.1 mm, and all experiments
were performed in atmospheric air. The impact velocity was varied from 0.36 m/s to 2.2 m/s thus varying the impact Weber number
from 5.5 to 206. The impacted liquid surface consisted of two fluids, namely water and methoxy-nonafluorobutane, C4F9OCH3 (HFE7100). The depth of the water and HFE7100 pool was varied from 2 mm to 25 mm. The collision dynamics of water in the
HFE7100 pool was observed to be drastically different from that observed for the water droplet impingement on a water pool.
The critical impact Weber number for jet breakup was found to be independent of liquid depth. Water–HFE7100 impact resulted
in no jet breakup over the range of velocities studied. Therefore, no critical impact Weber number can be defined for water–HFE7100
impact.
Received: 27 June 2001/Accepted: 29 November 2001 相似文献
9.
This work aimed at improving fine-scale measurements using cold-wire anemometry. The dissipation ɛ
θ
of the temperature variance was measured on the axis of a heated turbulent round jet. The measurements were performed with a constant current anemometer
(CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose allowed the use of the current
injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the
time constants obtained for two wire diameters −d=1.2 and d=0.58 μm – compare well with those measured at the same time using two other methods (laser excitation and pulsed wire). Moreover,
for these two wires, the estimated time constants were in good agreement with those obtained from a semi-empirical relation.
In the second part of the paper, a compensation procedure – post-processing filtering – was developed in order to improved
the frequency response of the cold-wire probes. The measurements carried out on the axis of the jet (Re
D
=16 500, Re
λ
≃ 167) showed that the frequency response of the 1.2 μm wire was significantly improved. In fact, the spectral characteristics
of the compensated signal obtained with the 1.2 μm wire compared fairly well with those from the 0.58 μm wire. Moreover, the
results indicated that the compensation procedure must be applied when the cut-off frequency of the cold-wire f
c is lower than two times the Kolmogorov frequency f
K. In the case where f
c ≃ 0.6f
K, the compensation procedure can reduce the error in the estimate of ɛ
θ
by more than 20%. When f
c ≃ 2f
K, the effect of the compensation is reduced to about 5%.
Received: 3 November 2000/Accepted: 23 March 2001 相似文献
10.
Roughness wall effects in a zero pressure gradient turbulent boundary layers were investigated using hot-wire anemometry.
The skewness and diffusion factors of u and v, the longitudinal and normal velocity fluctuations, were measured and represented using wall variables. The results indicate
that the wall roughness removes the crossover point between sweep and ejection events to the outer region of the layer for
a single Reynolds number Re
θ
> 3,000. This behaviour exhibits that the roughness surface favours the maintaining of sweep events obtained by a quadrant
analysis. These results show that communication between the wall region and outer region of a turbulent boundary layer exists
and the wall similarity hypothesis for a rough wall is questionable. The effect of the wall roughness on the position of the
point crossover from sweep to ejection motions with respect to the wall seems to be the same as that obtained when the Reynolds
number is higher.
Received: 8 March 2000/Accepted: 15 May 2000 相似文献
11.
Effect of local forcing on a turbulent boundary layer 总被引:6,自引:0,他引:6
An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent
boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from
a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re
θ
=1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f +≤ 0.044). The forcing amplitude is fixed at A
0=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the
forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise
vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are
examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy
is concentrated near the center of vortical structures.
Received: 17 March 2000/Accepted: 3 April 2001 相似文献
12.
This study investigates the enhancement of the laminar forced convection characteristics of backward-facing step flow in a two-dimensional channel through the installation of solid and slotted baffles onto the channel wall. The effects of the height of baffle H
b, inclination of baffle installation ϕb, height of slot in baffle H
t, inclination of slot in baffle ϕt, and distance between the backward-facing step and baffle D on the flow structure, temperature distribution and Nusselt number variation for the system at various Re are numerically explored. Results show that a slotted baffle can enhance the average Nusselt number for the heating section of channel plate by the maximum 190% when Pr=0.7, H
s=0.5, L=5, H
b ≤ 0.3, W
b ≤ 0.2, 0.1 ≤ D ≤ 0.5, 0° ≤ ϕb ≤ 45°, H
t ≤ 0.1, 0° ≤ ϕt ≤ 45° and 50 ≤ Re ≤ 400. As for the solid baffle, the enhancement may be up by 230%. The solid baffle might cause the re-separation of main stream, and consequently result in poor local heat transfer coefficient in the end region of heating section. This disadvantage can be obviously improved as the baffle is slotted. Besides the penalty of increase in pressure drop due to the baffle installation is much higher for the situation with solid baffle. 相似文献
13.
The lift force experienced by a spinning sphere moving in a viscous fluid, with constant linear and angular velocities, is
measured by means of a trajectographic technique. Measurements are performed in the range of dimensionless angular velocities
γ=aω/V lying between 1 and 6, and in the range of Reynolds numbers Re=2aV/ν lying between 10 and 140 (a sphere radius, ω angular velocity, V relative velocity of the sphere centre, ν fluid kinematic viscosity). A notable departure from the theoretical relationship
at low Reynolds number, C
L
=2γ, is obtained, the ratio C
L
/γ being found to significantly decrease with increasing γ and increasing Re. The following correlation is finally proposed to estimate the lift coefficient in the range 10<Re<140:
C
L
≅0.45+(2γ−0.45) exp (−0.075γ0.4
Re
0.7)
Received: 20 May 1996/Accepted: 9 November 1997 相似文献
14.
The Simha–Somcynsky (S–S) equation of state (eos) was used to compute the free volume parameter, h, from the pressure–volume–temperature (PVT) dependencies of eight molten polymers. The predicted by eos variation of h with T and P was confirmed by the positron annihilation lifetime spectroscopy; good agreement was found for h(P = constant) = h(T) as well as for h(T = constant) = h(P). Capillary shear viscosity (η) data of the same polymers (measured at three temperatures and six pressures up to 700 bars), were plotted as logη vs 1/h, the latter computed for T and P at which η was measured. In previous works, such a plot for solvents and silicone oils resulted in a “master curve” for the liquid,
in a wide range of T and P. However, for molten polymers, no superposition of data onto a “master curve” could be found. The superposition could be
obtained allowing the characteristic pressure reducing parameter, P*, to vary. The necessity for using a “rheological” characteristic pressure reducing parameter, P*R = κP*, with κ = 1 to 2.1 indicates that the free volume parameter extracted from the thermodynamic equilibrium data may not fully describe
the dynamic behavior. After eliminating possibility of other sources for the deviation, the most likely culprit seems to be
the presence of structures in polymer melts at temperatures above the glass transition, T
g. For example, it was observed that for amorphous polymers at T ≅ 1.52T
g the factor κ = 1, and the deviation vanish. 相似文献
15.
This study investigates the influence on the oscillating characteristics of a cavity shear layer by introducing either a
sloped bottom or a flow path modifier at the bottom of the cavity. All the experiments are performed in a recirculating water
channel. The laser Doppler velocimetry system and the laser sheet technique are employed to perform the quantitative velocity
measurements and the qualitative flow visualization, respectively. The Reynolds number, based on the momentum thickness at
the upstream edge of the cavity, is kept at about Re
θ
0=194 ± 3.4. It is found that, in addition to the feedback effect, the upstream moving part of the recirculating flow inside
the cavity also plays an important role in changing the oscillating characteristics of the unstable shear layer. As the bottom
of the cavity is either negatively or positively sloped, the oscillating characteristics of the cavity shear layer are modified
to different extents. Significant reduction of the oscillating amplitude within the cavity is found while the bottom slope
increases up to d/L=± 2/5. As the bottom slope further increases up to d/L=± 1/2, the self-excited oscillation is completely suppressed. In addition, the ability to suppress the self-excited oscillation
by the negative bottom slopes is superior to that in the case of a positive bottom slope. Depending upon the fence locations,
the upstream moving part of the recirculating flow will perturb the unstable shear layer at different x/L locations, leading to different oscillating amplitudes. The ability to promote the enlarged oscillating amplitude of the
unstable shear layer is better for a fence inclined at a positive angle than for one at a negative angle.
Received: 31 May 2000/Accepted: 11 January 2001 相似文献
16.
Experiments are presented on the surge effect induced by cylindrical bodies piercing a free surface at constant velocity.
The study covers the following ranges: 0.1 < Froude2 < 7.3, 0.7 < Weber < 89, 46 < Reynolds < 6000; with the Goucher number (Go) evolving between 1.8 and 4.5. Free-surface profiles are compared with those issued from a boundary element method (BEM)
simulation based on potential theory and axisymmetric flow configuration. The free-surface deformation is accurately predicted
by the BEM approach for Reynolds numbers (Re) higher than 500. However, for Re less than about 200, computed results underestimate the interface elevation, except on the axis of symmetry, where they remain
accurate. Finally, the magnitudes of the interface deformation predicted by the numerical simulation are provided over a wide
range of Goucher numbers (0.001 < Go < 50) both for hemispherical and conical tips. Implications of these results for phase detection probes are discussed.
Received: 18 December 2000/Accepted: 25 January 2001 相似文献
17.
Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re
δ
up to 104 using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled
from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer.
This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial
resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer.
The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re
δ
≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal
mixing before the transition point.
Received: 5 October 1999 / Accepted: 9 February 2001 相似文献
18.
Linear viscoelastic properties of SiO2/(AP/EP) suspension with various SiO2 volume fractions (ϕ) in a blend of acrylic polymer (AP) and epoxy (EP) were investigated at various temperatures (T). The AP/EP contained 70 vol.% of EP. The SiO2 particles were treated with epoxy silane coupling agent. The effects of the SiO2 particles are more pronounced in the terminal zone: a transition from viscoelastic liquid (ϕ ≤ 30 vol.%) to viscoelastic solid (ϕ ≥ 40 vol.%) was observed which can be interpreted as a critical gelation occurring at a critical particle content and critical
gel temperature. The SiO2/(AP/EP) systems exhibited a critical gel behavior at ϕ ≅ 35 vol.% and T ≅ 100°C characterized with a power–law relationship between the storage and loss moduli (G
′ and G
″) and frequency (ω); G
′ = G
″/tan(nπ/2) ∝ ω
n
. The critical gel exponent (n) was estimated to be about 0.45. The gelation occurred with increasing T. 相似文献
19.
The effect of mixing particles of different size on the electrorheological response under steady shear flow 总被引:2,自引:0,他引:2
The effect of mixing particles of different sizes on the electrorheological response of suspensions under steady shear flow
was investigated. Two sizes, 15 μm and 50 μm, of monodisperse spherical sulfonated poly(styrene-co-divinylbenzene) particles were used. Several electrorheological fluids were made containing different proportions of small
and large particles dispersed in silicone oil, but with constant overall particulate concentration. It was found that the
mixed size system produced the highest electrorheological response under the shear rates used (10 s−1 to 500 s−1), which is the opposite trend to previous studies of bimodal systems with larger size ratios.
Received: 21 December 2000 Accepted: 29 March 2001 相似文献
20.
An experimental wind-tunnel investigation was undertaken to determine the effects of Gurney flaps on a 40-deg cropped nonslender
delta wing at a chord Reynolds number of 250,000. In the experiment, the height of the Gurney flaps was varied from 0.01C to 0.05C, and the sideslip angle of the model was selected as 0, 5, 10 and 20 deg. In addition, the 0.05C Gurney flap was serrated with different heights of 0.01C to 0.05C separately. In comparison with the baseline clean configuration results, it was found that the model with plate Gurney flaps
can indeed increase the lift-to-drag ratio at moderate-to-high lift coefficients for the wing, and the greatest increment
was obtained for the 0.01C Gurney flap. The effect of Gurney flap on the increment of lift-to-drag ratio tends to be not significant with the increase
of sideslip angle. Moreover, the 0.05C serrated Gurney flap provides the best performance among the serrated Gurney flaps.
Received: 6 July 2000 / Accepted: 21 June 2001 Published online: 29 November 2001 相似文献